a) Field of the Invention
Gas molecules from a gas atmosphere formed of one or more gases or vapors can be absorbed by or given off into the gas atmosphere by optical coating systems. An absorption/emission equilibrium takes place depending on the pressure of the gas or the partial pressures of the gases. The varying degrees to which the different gases are absorbed and the influence of the gases on the optical characteristics of the coating system make it possible to deduce the partial pressure of an individual gas component.
b) Description of the Related Art
Most optical sensors based on this principle detect the change in intensity of the radiation reflected by or transmitted through this coating system as a measured quantity.
It has long been known, particularly for measurement of moisture, that optical coating systems formed of one or more dielectric single layers whose thickness is approximately equal to one fourth or one half of the wavelength of the measurement light are porous and change their reflection characteristics and transmission characteristics as a result of water absorption when there is a change in the moisture content of the surrounding air (see, e.g., Koch, "Optical Untersuchungen zur Wasserdampfsorption in Aufdampfschichten [Optical Investigations in Water Vapor Absorption in Evaporation Coatings]", phys. stat. sol., 12 (1965), pages 533-543).
DE 36 19 017 A1 proposes an optically dielectric moisture measurement device in which the optical coating system is arranged on a translucent carrier. The partially transmitting mirror formed in this way is placed in an optical beam path and a moisture-dependent measurement signal is formed by taking the quotient of the reflected and transmitted light intensities changing as a function of the absorption of moisture. The measurement signal formed in this way enables a highly sensitive detection of the change in the reflection behavior and transmission behavior of the optical coating system.
In all of the described solutions, a defined relationship is assumed between determined optical characteristics of the influenced optical coating system and diverse physical and chemical parameters of the medium acting on the coating system which allows a unique correlation between the measurement signal and the quantity of the parameter. Specifically, the partial pressure, especially the partial pressure of water vapor, is indicated as a parameter which can be determined.
The unique correlation of a measurement signal with a determined magnitude of partial pressure, especially partial pressure of water vapor, is sometimes highly prone to error.
The possibility of enabling measurements over a wide range of partial pressures of water vapor, especially in the low-moisture range, by means of defined optical coating systems lends increased significance to optical moisture sensors, e.g., for high-vacuum evaporation systems. The size of the moisture sensor is of secondary importance for the above-mentioned applications and, in addition, vacuum-tight fiber vias or lead-throughs are relatively expensive and are not available for ultra-high vacuum applications. Therefore, it is impossible to arrange the coating system directly on a fiber surface (DE 3832185 C2). Gluing the substrate to the end face of the fiber (DE 4133126 A1) is relatively complicated and makes exchange impossible for further cases of application and/or as a consequence of aging processes.
The sensor material of the present invention composes an oxidic or fluoridic function coating in which steam or another gas (or other "steams" such as alcohol vapor) which is condensable at low temperatures is included. The water inclusion is based on adsorption and is therefore reversible without any external action when the partial pressure of the steam decreases. The sensor material does not change.
The sensor of the present invention makes use of the change in the refractive index due to the included gas or vapor for detection. The sensor coat is applied as an interference layer. The reflection effect relies on interference; therefore, the reflection is highly wavelength-dependent (reflection minima, reflection maxima). A change in the index of refraction of the sensor coat causes a spectral displacement of the reflection and transmission extrema of the arrangement. The emission of gas is carried out when the partial pressure of the gas falls. Therefore, no heating or temperature monitoring is required for the operation of the sensor in a gas atmosphere with constant temperature.
The measurement of reflection and light attenuation described in U.S. Pat. Nos. 4,764,343 and 4,668,635 is based on the metallic characteristic of the sensor material which is lost when combined with gas. At a given partial pressure, there is a determined temperature at which the conversion of metal to the metal compound, and vice versa, takes place. The detection of partial pressure is carried out ultimately by measurement of the conversion temperature. The very high temperatures are therefore necessary for the operation of the sensor.
The sensor coating of our sensor is a component part of an interference layer arrangement. The special arrangement of the interference layers brings about an improved readout and increased detection speed.
The tempering possibility in our sensor improves the measurement precision in the measurement of systems with inhomogeneous temperatures or temperatures which change over time.
The capability of the function coating of absorbing gas molecules can be limited through contact with liquids or gases. In this case, reduced measuring sensitivity must be expected in spite of recalibration. The arrangements make it possible to simply replace the sensor. For measurements particularly in ultra high-vacuum systems or in clean gas systems, adhesive compounds are undesirable because gas evolution of the adhesive impairs the function of the system. The arrangement of the sensor ensures an inexpensive and adhesive-free sealing and flanging to the system.